1. Field of the Invention
This invention relates to a process and apparatus for forming tungsten silicide on a semiconductor wafer. More particularly, this invention relates to a process for forming tungsten silicide from a gaseous mixture comprising dichlorosilane and a tungsten-containing gas wherein constant gas flow conditions are maintained in a deposition chamber regardless of wafer diameter size.
2. Description of the Related Art
In the CVD processes for the depositions of materials, such as, for example, a tungsten silicide CVD process, on semiconductor wafers, the common practice is to provide a separate pattern of inlet gas flow for each size wafer to be processed, which to some extent, matches the diameter of the particular wafer. For example, for a wafer of approximately 200 mm. diameter (.about.8 inch), typically a 200 mm. (.about.7.92 inch) diameter gas inlet structure, commonly referred to as a showerhead, and generally illustrated at 2, in FIG. 1A, is typically provided with a 200 mm. (.about.7.92 inch) diameter pattern of 0.71 mm. (0.028 inch) diameter inlet holes, each spaced apart a distance of about 4.57 mm. (0.18 inches). When a wafer of approximately 150 mm. diameter (.about.6 inch) is processed, a showerhead of the same outer diameter is conventionally used, as generally illustrated at 4 in FIG. 1B, but the showerhead is now provided with a correspondingly smaller hole pattern (but with the same diameter individual holes) for the admission of process gas into the deposition chamber, even though the same size (inner diameter) chamber is used. Thus, for a 150 mm. diameter wafer, the gas inlet hole pattern is usually also about 150 mm. in diameter. Similarly, for a wafer of approximately 125 mm. diameter (.about.5 inch), the showerhead, generally illustrated at 6 in FIG. 1C, has the same outer diameter, but the showerhead is provided with a 125 mm. diameter hole pattern for the admission of process gas into the chamber, and an approximately 100 mm. (.about.4 inch) diameter wafer, as generally illustrated at 8 in FIG. 1D, is provided with a showerhead (of identical outer diameter) with a 100 mm. diameter pattern of inlet gas holes. Side by side comparisons of the gas flow patterns of the four different prior art susceptors shown in FIGS. 1A-1D shows the respective gas flows from the respective showerheads, as indicated by the arrows. It will be readily appreciated that both the gas flow pattern and total volume of gas entering the deposition chamber vary considerably with the difference in wafer diameter in the tungsten silicide deposition process of the prior art, even though the diameter and volume of the deposition chamber remained the same.
The reason for conventionally using such a different pattern of gas inlet holes for each diameter of wafers was to maintain a reactive zone directly above the wafer having outer regions corresponding to the edge of the respective wafers of varying diameters, and to keep the reactive gas concentration consistent over the wafer. It also was thought to be more efficient because if the same gas flow were to be used, for all sizes of wafers, most of the process gases flowed into the chamber for processing the smaller diameter wafers would be pumped out instead of deposited on the wafer.
It is also conventional, in such CVD processes, to use different diameter wafer supports, known as susceptors, for different diameter wafers. Such susceptors are provided, at least in some instances, with small raised portions or crowns on the upper surface of the susceptor which are generally arranged in a circle slightly larger than the diameter of the wafer being processed. The purpose of this pattern of crowns on the wafer support is to inhibit sideways movement or floating of the wafer during the deposition processing. Thus, for a 200 mm. (8 inch) diameter wafer, a susceptor having a diameter slightly larger than 200 mm. (8 inches), for example, from about 210 mm to 229 mm. (.about.81/4 to .about.9 inches), is utilized with a pattern of crowns thereon arranged in a circle having a diameter large enough to permit a 200 mm. (8 inch) diameter wafer to be mounted on the susceptor between the crowns. For a 150 mm. (6 inch) wafer, however, the larger susceptor is replaced in the deposition chamber with a smaller diameter susceptor having, for example, about a 165 mm. to about 178 mm. (.about.61/2 to .about.7 inch) diameter, with a pattern of crowns formed on its face permitting the mounting of a 150 mm. (6 inch) diameter wafer thereon between the crowns. Still smaller susceptors are then conventionally used for the processing of 125 mm. (5 inch)and 100 mm. (4 inch) diameter wafers.
FIGS. 2A-2D show side views, respectively, of the four different diameter prior art susceptors conventionally used for 100, 125, 150, and 200 mm. (4, 5, 6, and 8 inch) diameter wafers during CVD processing, with arrows illustrating the respective gas flow paths. In FIG. 2A, the susceptor used conventionally for processing a 200 mm. diameter wafer is shown at 10, having crowns 12 thereon to prevent lateral shifting of 200 mm. wafer 101 mounted thereon, and finger openings 14 through which fingers (not shown) may be inserted to lift wafer 101 off susceptor 10 to facilitate removal (or mounting) of wafer 101 from susceptor 10. As shown in FIG. 2A, a baffle plate 16 surrounds susceptor 10 and may be mounted to, or positioned adjacent, sidewall 52 of the processing chamber. Baffle plate 16 is provided with a central opening 17 therein which is from about 220 mm. to about 240 mm. (.about.8.65 to .about.9.5 inches) in diameter, to thereby define a gap 18 between susceptor 10 and baffle 16, of from about 5.1 mm to about 7.6 mm (.about.200 mils to .about.300 mils) to permit movement of the susceptor toward and away from the showerhead. Openings (not shown) may be provided in baffle 16 to permit the process gases to pass from showerhead 2 (shown in FIG. 1A) and the region above wafer 101 to flow to a vacuum outlet (not shown) on the side of the chamber.
Similarly, in FIG. 2B, the susceptor used conventionally for processing a 150 mm. (6 inch) diameter wafer is shown at 20, having crowns 22 thereon to prevent lateral shifting of 150 mm. diameter wafer 102 mounted thereon, and finger openings 24 through which fingers (not shown) may be inserted to lift wafer 102 off susceptor 20 to facilitate removal (or mounting) of wafer 102 from susceptor 20. As shown in FIG. 2B, a baffle plate 26 surrounds susceptor 20 and may be mounted to, or positioned adjacent, sidewall 52 of the processing chamber. Baffle plate 26 is provided with a central opening 27 therein which is sufficiently larger than the diameter of susceptor 20 to thereby define a gap 28 between susceptor 20 and baffle 26, similar in width to gap 18 in FIG. 2A, i.e., which also ranges from about 5.1 mm to about 7.6 mm (.about.200 mils to .about.300 mils) to permit movement of susceptor 20.
In FIG. 2C, the susceptor used conventionally for processing a 125 mm. (5 inch) diameter wafer is shown at 30, having crowns 32 thereon to prevent lateral shifting of 125 mm. diameter wafer 103 mounted thereon, and finger openings 34 through which fingers (not shown) may be inserted to lift wafer 103 off susceptor 30 to facilitate removal (or mounting) of wafer 103 from susceptor 30. As shown in FIG. 2C, a baffle plate 36 surrounds susceptor 30 and may be mounted to, or positioned adjacent, sidewall 52 of the processing chamber. Baffle plate 36 is provided with a central opening 37 therein which is sufficiently larger than the diameter of susceptor 30 to thereby define a gap 38 of the same width as gaps 18 and 28 between susceptor 30 and baffle 36 to permit movement of susceptor 30.
Similarly, in FIG. 2D, the susceptor used conventionally for processing a 100 mm. (4 inch) diameter wafer is shown at 40, having crowns 42 thereon to prevent lateral shifting of 100 mm. diameter water 104 mounted thereon, and finger openings 44 through which fingers (not shown) may be inserted to lift wafer 104 off susceptor 40 to facilitate removal (or mounting) of wafer 104 from susceptor 40. As shown in FIG. 2D, a baffle plate 46 surrounds susceptor 40 and may be mounted to, or positioned adjacent, sidewall 52 of the processing chamber. Baffle plate 46 is provided with a central opening 47 therein which is sufficiently larger than the diameter of susceptor 40 to thereby define a gap 48 between susceptor 40 and baffle 46 of the same width as gaps 18, 28, and 38 to permit movement of susceptor 40.
The reason for changing the diameter of the susceptors, when changing from a 200 mm. (8 inch) wafer to a 150, 125, or 100 mm. wafer, rather than using a standardized diameter for the respective susceptors was similar to the reasons for changing the showerhead hole pattern sizes for different size wafers, i.e., to maintain a gas flow/hot susceptor space consistent with the wafer diameter. Also for depositions on smaller wafers, by using a smaller susceptor, the cost of the susceptor could be reduced, there was less total deposition resulting in a shorter chamber cleaning time, and an increased throughput.
This use of different showerheads with differing hole patterns for the inlet gases flowing into a given size chamber provides a different inlet processing gas flow pattern for each size wafer being processed in the deposition chamber. Furthermore, such differences in inlet flow of the process gases, coupled with the use of susceptors of differing outer diameters and baffles of differing inner diameters, interposed between the inlet flow at one end of the deposition chamber and the outlet flow to a vacuum pump at the other end of the chamber, so that the annular gap between susceptor and baffle is located at a different place in the deposition chamber for each size wafer, results in a large variance in the conventional gas flow patterns in the deposition chamber between depositions on 100, 125, 150, and 200 mm. diameter wafers.
However, in the conventional deposition of materials such as CVD tungsten silicide depositions, such disparities in the flow rates and flow patterns used for the various diameter wafers has not been a problem. Apparently when forming tungsten silicide by a CVD process using a source of tungsten such as tungsten hexafluoride (WF.sub.6) and a source of silicon such as silane (SiH.sub.4), the activation energy of the silane gas is sufficient high to permit reaction of the silane with the tungsten hexafluoride on a repeatable and reliable basis despite such changes in gas flow rates and patterns when changing from a tungsten silicide deposition, for example, on a 200 mm. (.about.8 inch) diameter wafer to a 100 mm. (.about.4 inch) diameter wafer.
However, to improve the step coverage of the deposited tungsten silicide, as well as to lower the fluorine content in the tungsten silicide, dichlorosilane (DCS), having the formula SiH.sub.2 Cl.sub.2, has been used as a replacement gas for the silane used in the prior art processing. While this has, in some instances, improved the quality of the deposited tungsten silicide film, it has been found that the CVD process for the deposition of tungsten silicide, using DCS as the source of silicon, is very sensitive to such previously discussed changes in the gas flow rates and flow patterns present in the chamber during the deposition, resulting in the need to constantly readjust temperatures, pressures, and flow rates when changing from one diameter wafer to another. Such restrictions have, in turn, limited the optimization of the deposition uniformity when using DCS as the source of silicon.
It would, therefore, be desirable to provide a process and apparatus for the CVD formation of a tungsten silicide coating using a dichlorosilane source of silicon, which process could be used for wafers of differing diameters with little or no changes necessary in either processing parameters or hardware utilized.